Localized vacuum seals are well-known for use in the semiconductor processing industry. For an example of a local vacuum seal, see Petric U.S. Pat. No. 4,607,167 incorporated herein by reference in its entirety.
Charged particle beam lithographic machines typically include within contiguous vacuum regions a beam source, beam steering and forming elements, a stage for a semiconductor wafer or a mask (the workplace) and a mechanism to move the stage with respect to the beam. For electron beam lithography, the beam source is an electron beam source and the beam steering and forming elements are an electron optical column. For focused ion beam lithography the beam source is an ion beam source and the beam steering and forming elements are ion deflection and focusing elements.
It is known from the above-referenced patent document to include what is called there an envelope apparatus coupled to the lower end of the electron beam column. The tip of the envelope apparatus rests slightly above the surface of the workpiece which is a semiconductor wafer (or a mask blank) mounted on a carrier and moving stage. Very small gaps are maintained between the tip of the envelope and the surface of the wafer. These gaps form a graded (differentially pumped) vacuum seal when the machine is in operation. Since the envelope tip is a truncated circular cone, a radial seal is thereby formed. This advantageously allows the stage supporting the wafer to be outside the vacuum. Hence a relatively small sized vacuum enclosure is possible while keeping the electron beam within the vacuum, which is necessary for its propagation.
Within the vacuum envelope, concentric conical members establish discrete zones, each zone being in communication with an associated vacuum pump. The vacuum is graded from the pressure of the ambient atmosphere, to a low vacuum level produced by the first stage vacuum pump, to a higher vacuum level formed by a second stage vacuum pump, to the vacuum along the beam path formed by a high vacuum pump in communication with the central portion of the envelope. Outside the largest seal is a ring of air bearings that are supplied with compressed air, supplied at an appropriate pressure to produce an air bearing effect between the surface of the vacuum envelope and the surface of the wafer. Typically these air bearings are air jets spaced along a circle and supplied with air from an annular groove outside the vacuum.
Related disclosure is also found in Lamattina, et al., U.S. Pat. No. 4,584,479; Petric, et al., U.S. Pat. No. 4,524,261; Petric, et al., U.S. Pat. No. 4,528,451 and Young, et al., U.S. Pat. No. 4,818,838, all incorporated herein by reference in their entireties.
However, the present inventor has identified certain deficiencies in the above-referenced vacuum seals. The above-mentioned patent documents do not appear to recognize or mention one problem introduced by their vacuum seals. This problem typically occurs during loading and unloading of the workpiece substrate (mask or wafer). In this case, the substrate, which sits on a carrier (a portion of the stage), typically sits in a recess in the carrier. Ideally the carrier is configured so that the depth of the recess is exactly equal to the thickness of the substrate, so the top surface of the substrate is exactly at the same level as (coplanar with) the upper surface of the carrier outside the recess. However, even with this arrangement, a vacuum leak develops when the gap between the edge of the substrate and the carrier traverses the vacuum seal due to movement of the stage on which the carrier rides. This is because inevitably the substrate cannot fit exactly against the corresponding edge of the recess in the carrier. During the time this gap traverses the vacuum seal, the annular regions of the seal are partially vented. In other words, the gap is a leak that introduces a large quantity of undesirable air into the vacuum portions of the seal. This of course undesirably allows the atmospheric air to enter into the electron beam column thus possibly contaminating the column, and at a minimum requiring a certain amount of time to reestablish the proper vacuum level after the gap is traversed.